The invention relates to overhead traveling cranes, and more particularly, to a lifting arrangement for overhead traveling cranes.
Powerhouse or class xe2x80x9cAxe2x80x9d overhead traveling cranes are generally utilized for maintenance of power producing equipment, such as generators and turbines. The number of lifts a powerhouse crane performs over its life span is very small, however, the capacity and lift height of the lifts that are performed are extreme. Because the duty cycle of the crane is very low, the lift speeds tend to be slow, especially when the load is heavy. The work done by the crane requires very good control and spotting ability to ensure proper procedure. True vertical lift is also required for ease of assembly of very large machine parts and assemblies.
A powerhouse crane typically includes a long, large diameter drum that is selectively rotated by a motor. The drum is coupled to the motor by a large, high ratio gearbox. A rope wound around the drum winds on to and off of the drum in response to rotation of the drum in opposite directions. Typically, the rope is wire rope and the drum has a double helical groove in which the rope is double reeved as the rope winds on to the drum. A bottom block is supported by the rope such that the bottom block moves up and down as the rope winds on to and off of the drum.
Powerhouse cranes generally include a large amount of rope because of the extreme lift heights and the use of a double reeved rope configuration to provide true vertical lifting. The drum needs to be sized to store this rope. Generally, the length and/or the diameter of the drum can be increased to add rope storing capacity. Both solutions result in separate problems. As the length of the drum is increased, the length of the trolley frame that supports the drum must also be increased. A longer trolley frame experiences greater bending moments, and therefore, the load members of the trolley frame must be increased in size to compensate. As the diameter of the drum is increased, so does the amount of torque which is required to turn the drum. Typically, a larger gearbox is necessary to provide more torque.
The costs associated with providing enlarged drums, gearboxes and trolley frames add significantly to the overall price of the crane. The components often need to be custom designed for each application, thereby resulting in the manufacturing of only a single crane at a time. Use of mass produced components could significantly reduce the overall cost of these cranes.
Accordingly, the invention provides a crane that can utilize mass produced drums, trolley frames and gearboxes. The invention includes the use of two lift trains. Each lift train includes a drum that is single reeved together with the drum of the other lift train. In some embodiments, both lift trains include similarly sized drums, gearboxes and motors where. The components of each lift train are generally smaller than those typically used on powerhouse cranes. Because the costs associated with the components increase exponentially with the size and torque requirements, the cost of two smaller lift trains is less expensive than the cost of a single custom built lift train.
For lifts of a similar height, a lifting arrangement that includes a single reeved drum generally requires half as much rope as a lifting arrangement that includes a double reeved drum. Although the invention utilizes a lifting arrangement that includes a single reeved drum, the overall amount of rope utilized is similar to that of a lifting arrangement that includes a double reeved drum because the invention utilizes two drums that are singly reeved together. However, the use of two drums reduces the amount of rope stored on each drum by half. Accordingly, the length and/or diameter of each drum can be dramatically reduced when compared to the drums typically utilized on powerhouse cranes. When the length of the drum is reduced, the length of the trolley frame is reduced resulting in the ability to use a mass produced trolley frame, such as the trolley frame disclosed in U.S. Pat. No. 5,992,730, which is assigned to the assignee of this application. When the diameter of the drum is reduced, the torque requirements are reduced resulting in the ability to use a smaller mass produced gearbox. A ring gear external to the gearbox may be utilized to increase the torque of a smaller gearbox such that very high ratios (e.g., ratio of 600 to 1) can be achieved with a standard three stage helical gearbox.
Although the cost of components utilized on a powerhouse crane can be reduced as discussed above, the components must provide a lifting arrangement that meets all safety requirements including fleet angle requirements. In one embodiment, the invention provides a lifting arrangement that meets all fleet angle requirements by staggering the axial position of each drum in relation to the other drum. The fleet angles in the full up and full down positions can be equalized by this positional shift between the two drums to optimize the fleet angles and thus maximize rope life. A bottom block with two separate sheave nests can also be utilized to optimize the rope fleet angles. Use of two separate sheave nests allows for optimum placement of the sheave nests with respect to the corresponding drum. Additionally, the width of the bottom block can be increased such that the sheave nests can be placed at any location with respect to the drums. Generally, the sheave nests are located near the ends of the bottom block. In some embodiments, a combination of drum stagger and custom bottom block length are used to ensure proper fleet angles. Adjustment of the drum stagger and bottom block length can be used to solve the fleet angle limitations of a crane including any combination of variables.
As is apparent from the above, the invention provides a lifting arrangement for a crane that includes the use of dual drums single reeved together to provide true vertical lift. The invention also provides a lifting arrangement that includes mass produced components. The mass produced components provide a crane having a smaller footprint and height when compared to previous cranes built for similar applications. The invention also provides a lifting arrangement that allows for equalization of full up and full down rope fleet angles through utilization of axially staggered dual drums and/or an extra wide bottom block with two separate sheave nests. The invention also provides a lifting arrangement that includes commercially available components which reduce the overall cost of the crane when compared with current powerhouse crane designs, while still meeting all powerhouse crane requirements. Other objects of the invention will become apparent to those skilled in the art upon review of the following detailed description and drawings in which like numerals are used to designate like features.